Power can be from any 15-18v supply. I use an old 12v/1A wall
transformer (outputs about 17v with no load.)

How it works:

This circuit controls both the current and the charge voltage.
U1 along with resistors R2, R3 and R4 generate a 13.4v output.
This is the voltage specified by gell cell manufacturers as a
"constant charge voltage." At 13.4v the batt will never
overcharge. Some batteries specify 13.6 to 14.7 volts, but testing
has shown that many batts will leak if float charging at this
voltage *. U1 can only carry up to 1.5 amps and is limited by
its heat dissipation. Q1 and R1 limit the current to about 0.65
amps. This limits the current to protect U1 and the battery from
over current.

* Panasonic batteries may be able to withstand this voltage.
My work with UPS designs has had terminals of several brands corrode
off of batteries if the voltage was set higher than 13.4v.

Notes:

Input voltage can be from 15-20v.

Q1 can be any general-purpose NPN transistor.

All resistors are 1/4 watt except for R1, which is 1/2W or
greater.

C1/2 are any 0.1uf capacitors.

A 7Ah Gel Cell will be close to fully charged in a few hours
after a normal days use. Leaving the battery on the charger will
NOT overcharge it. In fact it will maintain a battery forever.

Assembly:

The entire circuit can be constructed on a small perf board
1" square or so. U1 (LM317) must have a heat sink; a small
piece of aluminum will do. There are many heat sinks available.
The size of the heat sink depends on the input voltage. Note:
the case of U1 is connected to pin 3, so the heatsink must be
isolated from any other parts of the circuit. An insulator (TO-220
type) can be used to isolate the case from the heatsink if needed
(i.e. bolting U1 to the case as a heatsink.) U1 should not get
too hot to touch.

Adjust R4 for 13.4 to 13.5 volts out with no load.

Parts (Radio Shack part numbers given.):

Description

RS part #

U1

LM317T adj. regulator

276-1778

Q1

2N2222A NPN trans.

276-2009

R1

1 ohm/ 1/2 watt resistor (note 1)

271-131

R2

2200 ohm 1/4 w, 5% res

271-1325

R3

470 ohm 1/4 watt, 5% res

271-1317

R4

4.7K trim Pot

271-281

C1,2

0.1uf/5ov ceramic cap

272-135

Heat Sink

276-1368

Mounting hardware (note 2)

276-1373

Notes:

1. RS part is 10 watt, only size they carry in 1 ohm.
2. Required if heatsink is isolated.
3. The current limit can be changed as needed. Current is set
by: I=0.65/R1 R1 must be able to dissipate at lest 0.423/R1 watts.
4. The time to charge will be sped up by setting a higher voltage.
13.6 is usually fine. 13.4 was chosen to allow infinite charge
time.